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In a Dáil reply, Finance Minister Michael Noonan said the level of expenses reimbursed tax free "was found to be excessive and several cases included expenses which had not be incurred at all or were personal rather than business related."

The reply was in response to a question from Fianna Fáil's Michael McGrath.

"In many instances contractors will have acted on professional advice and the people who provided that advice had serious questions to answer," Deputy McGrath said.

"In light of these startling figures any self employed contractor providing services through a company under contract to a third party client would be well advised to have their tax situation independently checked to ensure they are not building up a massive bill of underpaid tax," he advised.

Since July 2013, a total of 385 audits have closed with additional tax due in 299 cases while a further 49 have agreed to settlements. A total of €9.3m in interest tax and penalties has been collected.

368 companies remain under audit.

This follows an earlier Revenue pilot project in the South West region, which yielded €4.5m.